1. Field of Invention
This invention relates to Global Positioning (“GPS”) receivers, and in particular to high-sensitivity GPS receivers for use with wireless networks.
2. Related Art
The worldwide utilization of wireless devices such as two-way radios, portable televisions, Personal Digital Assistants (“PDAs”) cellular telephones (also generally known a “mobile phones” and/or “cell phones”), satellite radio receivers and Global Positioning Systems (“GPS”) is growing at a rapid pace. Cellular telephones, including Code Division Multiple Access (“CDMA”), Global System for Mobile Communication (“GSM”), and Personal Communication System (“PCS”) devices, have become commonplace in today's society. The use of these and other wireless devices to provide voice, data, and other services, such as Internet access, has provided many conveniences to cellular system users. Additionally, the number of features and applications offered by many wireless devices and wireless service providers is increasingly matching, and in many cases outpacing, the features offered by traditional land-line telephone devices and service providers. Features such as call waiting, call forwarding, caller identification (“caller I.D.”), three-way calling, data transmission and others are commonly offered by both land-line and wireless service providers. These features generally operate in a similar manner on both wireless devices and land-line telephones. Additional features and applications such as “free” long distance, Internet access, electronic gaming, built-in cameras, personal data assistants (“PDAs”), and the like make wireless communication devices a popular choice among many consumers.
Furthermore, the usage of other wireless communications systems, such as two-way paging, trunked radio, Specialized Mobile Radio (“SMR”), by police, fire, and paramedic departments, has also become common for mobile communications. The explosive growth of wireless communications systems has not stopped there. Emergency response teams use wireless communications to stay in contact with hospitals, fire departments and police departments. Taxicabs, buses, delivery trucks and even tractor trailers use wireless communications to dispatch services, track vehicles and monitor routes. School buses in some communities even use wireless communications to automatically alert parents when they are within a few minutes of a pick-up point. Businesses large and small use wireless communications to deliver services, monitor inventory and manage workforces.
GPS systems (also known as Satellite Positioning System “SPS” or Navigation Satellite System) have also become commonplace. In general, GPS systems are typically satellite (also known as “space vehicle” or “SV”) based navigation systems. Examples of navigation systems include but are not limited to the United States (“U.S.”) Navy Navigation Satellite System (“NNSS”) (also know as TRANSIT), LORAN, Shoran, Decca, TACAN, NAVSTAR (an older name for GPS), the Russian counterpart to NAVSTAR known as the Global Navigation Satellite System (“GLONASS”) and the future Western European proposed “Galileo” program. As an example, the US NAVSTAR GPS system is described in GPS Theory and Practice, Fifth ed., revised edition by Hofmann-Wellenhof, Lichtenegger and Collins, Springer-Verlag Wien New York, 2001, which is fully incorporated herein by reference.
Typically, GPS receivers receive radio transmissions from satellite-based radio navigation systems and use those received transmissions to determine the location of the GPS receiver. The nominal GPS operational constellation generally comprises 24 satellites, each of which make a complete orbit of the earth in 12 hours. There are often more than 24 operational satellites as new ones are launched to replace older satellites. The ground track (path followed by the satellite over the ground as the earth rotates beneath it) of each satellite orbit repeats almost exactly, once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four satellites in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight satellites visible from any point on the earth.
Generally, each GPS satellite in a GPS satellite-based radio navigation system broadcasts a radio transmission, that contains its location information, and orbit information. More specifically, the information contained in the signal broadcast by the GPS satellite includes time-tagged data bits marking the time of transmission of each of a plurality of subframes at the time they are transmitted by the satellite, a data frame being transmitted every thirty seconds. Three six-second subframes contain orbital and clock data. Satellite Clock corrections as well as precise satellite orbital data sets (ephemeris data parameters) for the transmitting satellite are sent in multiple subframes. Additional subframes transmit different pages of system data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period.
As an example, each of the orbiting GPS satellites in the United States GPS system contains four highly accurate atomic clocks: two Cesium and two Rubidium. These clocks provide precision timing pulses, which are utilized in generating two unique binary codes (also known as a pseudo random noise “PRN,” or pseudo noise “PN” code), that are transmitted from the GPS satellites to Earth. These PN codes identify the specific GPS satellite in the GPS constellation.
Each GPS satellite also transmits a set of digitally coded ephemeris data that completely defines the precise orbit of the GPS satellite. The ephemeris data indicates where the GPS satellite is at any given time, and its location may be specified in terms of the GPS satellite ground track in precise latitude and longitude measurements. The information in the ephemeris data is coded and transmitted from the GPS satellite providing an accurate indication of the exact position of the GPS satellite above the earth at any given time. It is appreciated by those skilled in the art that the location of the GPS receiver may be determined by applying the well-known concept of intersection utilizing the determined distances from the GPS receiver to three GPS satellites that have known GPS satellite locations.
The GPS receiver produces replicas of a coarse acquisition (“CA”) code sequence for a specific satellite with some form of a CA code generator. PRN codes are defined for 32 satellite identification numbers. Some GPS receivers may store a complete set of precomputed CA code chips in locally, but a hardware implementation, such as a shift register, can also be used. The CA code generator produces a different 1023 chip sequence for each phase tap setting. In a hardware implementation, the code chips can be shifted in time, for example by slewing the clock that controls the shift registers. In an implementation where the codes are stored locally, the required code chips are retrieved from memory.
The CA code generator repeats the same 1023-chip PRN-code sequence every millisecond. The receiver adjusts the code replica in time until it finds a match, or correction, with the PRN code received from the satellite. Once the codes begin to align, signal power levels begin to be detected. When complete correction is achieved, a spread-spectrum carrier signal is de-spread and full signal power is detected.
A phase locked loop (“PLL”) that can lock to the signal is used to demodulate a navigation message from the GPS carrier signal. The same loop can be used to measure and track the carrier frequency and any associated Doppler shift. By keeping track of the changes to a numerically controlled oscillator, carrier frequency phase can be tracked and measured as well.
The position of the receiver is determined as the point where the pseudo-ranges from a set of GPS satellites intersect, and is found from multiple pseudo-range measurements at a single measurement epoch. The pseudo range measurements are used together with estimates of satellite positions determined by the ephemeris data received from each satellite during the measurement. The ephemerides allows the receiver to compute the satellite positions in three dimensions at the point in time they sent their respective signals. Four satellites can be used to determine three position dimensions and time. Position dimensions are typically computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (“ECEF XYZ”) coordinates.
The time measurement is used to correct any offset occurring within the clock in the receiver. Consequently, in most applications, a relatively inexpensive clock can be implemented in the receiver, saving on the cost of the bill of materials for the receiver device. Receiver position can be computed from the determined satellite positions, the measured pseudo-ranges (corrected for clock offsets, latency introduced from ionospheric delays, and relativistic effects), and a receiver position estimate. If a more optimum receiver clock is provided, three satellites can be used to determine three position dimensions. In practice, however, this is rarely achievable, and four satellites are used to compute a three dimensional fix with the user clock error removed, or three satellites are used to compute a two-dimensional, horizontal fix (in latitude and longitude) given an assumed altitude.
With the growing widespread use of these technologies, current trends are calling for the incorporation of GPS services into a broad range of electronic devices and systems, including PDAs, cellular telephones, portable computers, radios, satellite radios, trucked radio, SMR, automobiles, two-way pagers and the like. At the same time, electronic device manufacturers constantly strive to reduce costs, enhance performance, augment feature sets, extend battery life, and produce the most cost-attractive product possible for consumers.
In cellular telephony, the interest of integrating GPS receivers with cellular telephones stems from a new Federal Communications Commission (“FCC”) requirement that cellular telephones be locatable to within 50 meters once an emergency call, such as a “911” call (also referred to as “Enhanced 911” or “E911”) is placed by a given cellular telephone. When emergencies occur, people are accustomed to dialing 9-1-1 (normally referred to as a “911” call) on a land-based (also known as “land-line”) telephone via the public switched telephone network (“PSTN”), which contacts an emergency response center that is able to automatically identify the location of the land-based telephone where the call originated.
Unfortunately, because wireless devices, such as cellular telephones, are not hard wired into the PSTN and can move about, the emergency response center cannot determine their location as is possible with automatic-number-identification (“ANI”) techniques available for use with land-line calls. Furthermore, conventional wireless devices without a position determination capability are unable to automatically determine and communicate their location without a person actively entering or describing their location. In response, the United States Congress, through the FCC, has enacted a requirement that cellular telephones be locatable to within 50 meters once an emergency mobile telephone call, such as an E911 call, is placed by a user on a given cellular telephone. This type of position data would assist police, paramedics, and other law enforcement and public service personnel, as well as other agencies that may need to have legal rights to determine the position of specific cellular telephone. The E911 services, however, operate differently on wireless devices than a 911 call does on land-line telephones.
When a 911 call is placed from a land-line telephone, the 911 reception center receives the call and determines the origin of the call. In case the caller fails, or forgets, to identify his or her location, the 911 reception center is able to obtain the location from which the call was made from and send emergency personnel to the location of the call. Thus, even if the caller is unable to give his or her position, emergency personnel can be dispatched to the location from which the call was made.
If instead, an E911 call is placed from a wireless device such as a cellular telephone, the E911 reception center receives the call but cannot determine the origin of the call without information provided by the person who made the call. If the caller fails to, forgets to, or simply cannot identify his or her location, the E911 reception center is unable to determine the precise location of the call because the wireless device is only tied to the PSTN at a central location, such as a base-station, for example. At present, the best that the E911 reception center may do is to determine the location of the cell site from which the call was placed. Unfortunately, typical cell sites in a wireless network system may cover an area with approximately a 30-mile diameter. Further refinement of the location may be determinable in a digital network by the power setting of the calling wireless device. But, this still results in an area covering multiple miles. Therefore, emergency personnel cannot be dispatched to the scene without more information.
A proposed solution to this problem includes integrating GPS receivers with cellular telephones. An added benefit to this proposed solution is that any position or position-related data produced by an integrated GPS receiver may be utilized by the cellular telephone user for directions, latitude and longitude positions (locations or positions) of other locations or other cellular telephones that the cellular user is trying to locate, determination of relative location of the cellular user to other landmarks, directions for the cellular telephone user via internet maps or other GPS mapping techniques, and the like. Such data may be of use for other than E911 calls, and would be very useful for cellular and PCS subscribers.
As an example of the current thrust to integrate GPS receivers with cellular telephony, U.S. Pat. No. 5,874,914, issued to Krasner, which is incorporated by reference herein in its entirety, describes a method wherein a base station (also known as a base station and/or the Mobile Telephone Switching Office “MTSO”) transmits GPS satellite information, including Doppler information, to a remote unit (such as cellular telephone) utilizing a cellular data link, and computing pseudoranges to the in-view GPS satellites without the handset having to receive GPS satellite ephemeris information directly from the satellites. Another patent that concerns assistance between the GPS system and wireless networks is U.S. Pat. No. 5,365,450, issued to Schuchman, et al., which is also incorporated by reference herein in its entirety.
The integration of GPS capabilities with a wireless communication device, however, presents some challenges. For example, due to their mobile nature and because mobile-device designers are striving for smaller packaging as well as longer battery life, wireless communication devices often find themselves in areas where the signal strength of received GPS signals is weaker than desired. As a result, it may be a challenge for the GPS receiver to receive and lock onto the GPS signal received from the GPS satellites.
Therefore, there is a need in the art for a GPS receiver that can receive and recognize signals from the GPS constellation in situations where signal strength is weaker than desired.